Spray paint gas simulation
I would use both a smoke sim and a particles sim. 

You can use a smoke sim on the actual particle system. This is what I did here.

I used the particle system for the little tiny paint drops and of course the smoke for the mist.  






Create a circle with a face (as the nozzle for spray)
Create an icosphere to use for the Particles.
Add a particle system to your circle
Make sure you choose your icosphere as the object to emit. 
Create a big box for your Smoke sim, make sure it covers as much as you need reach the stencil and wall. 
Add a wind Force Field in the direction you want your paint to blow.
Make sure your wall is set as a collision object and have it set as a Canvas for dynamic paint.
Choose your circle emitter and make sure you choose for the smoke to emit from the particles. I have a screen shot below how I set it up. You will obviously need to change the settings to better match your scene but that is the gist of it.